1. Field of the Invention
The present invention relates to rigid foams, and more particularly to the preparation of such foams utilizing fluorocarbon blowing agents.
2. Description of the Prior Art
Rigid foams conventionally have been prepared by a two-component or three-component process. One component, generally referred to as component A, comprises isocyanate. The second component, known generally as component B, comprises any of various polyols, particularly polyester or polyether polyols. Component B may also comprise a surfactant, a catalyst package and a blowing agent, any of which may be introduced to the reaction mixture as a third stream. If an excess of isocyanate is employed, foams may be formed that are modified polyisocyanate foams. The foams produced by these standard methods commonly have a density in the range of of from about 1 lb.ft..sup.3 to about 4 lb./ft..sup.3, have a closed cell content on the order of about 94% and have low friability characteristics.
Ordinarily, the polyol is mono- or di-functional. That is, the polyol molecule has one or two branches which provide linking sites for cross-polymerization. Typically, the blowing agent is Freon-11 or Freon-12 (trade designations of E. I. du Pont de Nemours & Co.) or a mixture thereof, often in combination with water.
Recently, however, such processes for preparation of rigid polyurethane foam have been the subject of environmental concern in view of the deleterious effects Freon-11 and Freon-12 have been reported to have on the earth's ozone layer. As a result, the availability of Freon-11 and Freon-12 for such use is seriously threatened and the total phasing out of the use of Freon-11 and Freon-12 in the manufacture of rigid foam is desirable. Freon-22 (a trade designation of E. I. du Pont de Nemours & Co. for monochlorodifluoromethane or CHClF.sub.2) has been considered as a blowing agent in rigid foams (see, for example, U.S. Pat. No. 4,205,136 and 4,636,529), usually in combination with other freon blowing agents. However, Freon-22 generally has been considered and found to have a vapor pressure so high that the use of significant quantities of it as a blowing agent in standard formulations results in a closed cell content too low to be acceptable for typical rigid foam applications.
Other problems have also been encountered with rigid foams based on, for example, Freon-11. In particular, foam based on Freon-11 and having a density in the range of 1.8 to 2.5 lb./ft..sup.3 and prepared without use of aromatic polyethers tend to shrink at low temperatures. In fact, at temperatures of, say, -75.degree. F. or lower, conventional Freon-11 foam has been found to have poor dimensional properties.
Accordingly, methods for preparation of rigid polyurethane foam are needed that do not utilize Freon-11 or Freon-12, but which are economically competitve with foams made with Freon-11 or Freon-12 and produce a foam that has a density equivalent to that of conventional foams, a closed cell content at least as high as that of conventional foams and low friability characteristics. It it also desirable that the foam resist shrinkage at low temperatures.